The present invention relates to electrical transformers, and particularly to voltage surge or impulse protection for dual voltage distribution transformers.
Dual voltage distribution transformers are generally understood in the art as being those transformers having a dual voltage rating, typically a high voltage rating and a low voltage rating. Such transformers are capable of operating at either rating simply by connecting a plurality of primary, typically high voltage windings in series to effect the high voltage rating or in parallel to effect the low voltage rating. To conveniently convert from one voltage rating to the other, a switch, commonly referred to as a series-multiple switch or a dual voltage switch, is utilized. For example, a transformer may be equipped with a plurality of 7,200 volt primary windings, which if connected in parallel provide a 7,200 voltage rated transformer. On the other hand, if these plural primary windings are connected in series, the transformer's voltage rating is 7,200 volts multiplied by the number of primary windings so connected.
A typical application for dual voltage distribution transformers is in distribution circuits that are slated for eventual voltage uprating. The utility installs the transformer with its primary windings connected in parallel to establish the low voltage rating. Customers are served at this low voltage rating while the distribution circuit is prepared for voltage uprating. This involves changing out other distribution transformers, re-spacing distribution conductors, installing lightning arresters rated for the eventual higher distribution circuit voltage, etc. While voltage uprating the dual voltage distribution transformer is effected quickly simply by operation of the dual voltage switch, preparing the remainder of the distribution circuit for voltage uprating may take months, even years. In the meantime, if the dual voltage transformer, while connected in accordance with its lower voltage rating, is insulated at the basic impulse level (BIL) consistent with the voltage ratings of the individual primary windings but is protected by external lightning arresters rated in terms of voltage protection level according to the eventual higher distribution circuit voltage, the parallel connected primary windings are extremely vulnerable to damage by lightning induced surges. Consequently, to protect dual voltage distribution transformers during this vulnerable period, the high voltage primary windings must be insulated to a higher basic impulse level to safeguard the transformer up to the higher surge arrester protection level corresponding to the eventual higher voltage rating. This involves placing extra insulation between the turns or layers of the high voltage primary windings; such extra insulation being unnecessary when the primary windings are series connected to establish the higher transformer voltage rating assuming a properly rated external surge arrester has been installed.
In commonly assigned, copending application entitled "Distribution Transformer With Surge Protection Device", Ser. No. 673,214, filed Nov. 20, 1984, the disclosure of which is specifically incorporated herein by reference, plural arrester elements are located within the transformer tank and respectively electrically connected across sections of a single high voltage primary winding of a distribution transformer to afford enhanced voltage surge protection. It has been found that this arrangement provides unique primary winding protection against voltage surges entering the transformer via the low voltage secondary winding terminals, as well as via the high voltage primary winding terminals. This approach is a significant improvement over the traditional approach to distribution transformer surge protection of connecting external arresters across the primary winding terminals.
It is accordingly an object of the present invention to provide improved voltage surge protection for dual voltage distribution transformers.
An additional object is to provide full voltage surge protection for dual voltage distribution transformers while connected in either their higher or lower voltage rating configurations.
A further object is to provide a voltage surge protected, dual voltage distribution transformer which does not require the change out of external surge arresters when the transformer's voltage rating is changed, thus minimizing customer service interruptions.
Yet another object of the present invention is to provide a dual voltage distribution transformer of the above-character which is protected against voltage surges without the necessity of installing external arresters.
An additional object is to provide a dual voltage distribution transformer of the above-character wherein the levels of voltage surge protection can be closely tailored to the voltage ratings of the transformer, thus permitting reductions in the insulation levels to which the transformer must be designed.
Another object is to provide a dual voltage distribution transformer of the above-character which is internally equipped with a multi-rated surge arrester whose voltage protection level is changed coincidentally with changes in the transformer's voltage rating.
Still another object is to provide a dual voltage distribution transformer of the above-character which is of a cost improved construction, efficient in operation and reliable over a long service life.
Other objects of the present invention will in part be obvious and in part appear hereinafter.